I will admit that html+css is a relatively good (set of) language(s) to describe a UI, and JS is the way to tie the dynamism together and provide client-side logic. But is it better than just learning the native version for each platform?
My sense is that it's surprising we still don't have a decent way to build native apps in JavaScript. Titanium looked like the answer but I have trouble following where they're going. NativeScript looks promising so far but cannot figure out the tie-in to Telerik.
Btw, you may need an addition for Swift, I believe there is at least one cross platform tool for it now.
As for the general concerns, i can add that it's not just 'i know JavaScript and don't want to learn anything else', but rather having some common ground while building products/services on the web. It's probably ok for a big company to support say 3 platforms (web, android & ios), but hardly an option for a small startup or a one man band. Now i'd be more than happy to just stick with web, or at least the solutions bringing the others closer to it (that's by the way where Titanium is pretty weak).